Much effort has been directed in the past towards the provision of silver halide photographic materials which have a high photographic sensitivity, which produce little fog, and with which the change in photographic characteristics with the passage of time on storage is slight.
For example, organic thioether compounds have long been used as silver halide solvents or chemical sensitizing agents in the manufacture of silver halide photographic emulsions for increasing photographic sensitivity.
For example, techniques for the manufacture of so-called monodispersed silver halide photographic emulsions which have a uniform silver halide grain size by including organic thioether compounds during the precipitation or physical ripening processes in the manufacture of the silver halide photographic emulsions (referred to hereinafter simply as emulsions) have been described, for example, in U.S. Pat. Nos. 3,271,157, 3,531,289, 3,574,628 and 4,057,429.
Furthermore, techniques for increasing the photographic sensitivity of emulsions by including organic sulfur compounds during chemical ripening during the preparation of the emulsion or immediately prior to coating have been described, for example, in U.S. Pat. Nos. 2,521,926, 3,021,215, 3,038,805, 3,506,443, 3,057,724, 3,062,646, 3,574,709, 3,622,329 and 3,625,697.
Furthermore, a technique for raising the photographic sensitivity of emulsions using tellurium compounds which contain a tellurium atom, preferably substituted on an aromatic ring, such as bis(p-ethoxyphenyl)telluride, during the formation of silver halide grains has been disclosed in No. JP-A-53-57817 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application").
However, there is a tendency towards increased fogging when organic thioether compounds are included during the preparation of an emulsion.
Techniques such as carrying out the precipitation at a pH value of less than 4, or using highly refined organic thioether compounds are known for preventing the fogging which arises when organic thioethers are used, but these results are often unsatisfactory and there is a further problem in that such techniques are not practical on an industrial scale.
Furthermore, thioether compounds which may be used in combination with certain specified antifogging agents, as disclosed in No. JP-B-58-27489, and thioether compounds such as those disclosed in No. JP-B-58-30571 have been developed, and the development of compounds which give rise to less fogging and with which high sensitivity could be achieved was anticipated, but these objectives have not yet been realized with thioether compounds (the term "JP-B" as used herein refers to an "examined Japanese patent publication").
Furthermore, the organic telluroether compounds disclosed in No. JP-A-53-57817 certainly have a sensitivity raising action but, as shown in the examples, the effect is inadequate and there is a marked deterioration in photographic characteristics in respect of photographic sensitivity and gradation, for example, with the passage of time on storage.